Project Summary/Abstract Marijuana laws regarding the sale and possession of recreational marijuana in the U.S. have changed substantially in the past five years, making the drug more accessible through legal purchase to state residents aged 21 and older. Yet, as more states consider legalization, an essential policy question still needs to be addressed: Is a young person more likely to use marijuana if the drug is now more available to them through legal purchase? Studies that have examined trends in use over time have attempted to answer this question, but they have failed to account for individual changes in use over time. Moreover, any examination of whether increased usage can be explained by changes in a state?s legal policies needs to account for increased access to the drug via availability of stores where one can legally purchase recreational marijuana (i.e., marijuana outlets). The density of outlets around one?s home and proximity to these outlets need to be taken into account, as well as whether there are clear signage and advertisements from storefront signs and billboards in the immediate area that identify the outlet as selling recreational marijuana. We propose a time sensitive and unique research study that seeks to provide the most compelling data to date to answer the policy question regarding whether proliferation of recreational marijuana outlets in a large populated area influences young adult marijuana outcomes such as use, consequences, intentions to use, and beliefs about the drug. We propose to utilize address and marijuana outcome data from a multi-wave ongoing study with young adults in Los Angeles County, where one timepoint captures marijuana use from the sample prior to the opening of recreational outlets (pre-January 1, 2018) and a second timepoint one year later occurs after outlets began opening in the County for legal purchase (post-January 1, 2018). We plan to utilize an innovative data collection approach we developed to capture information about locations and signage/advertisements of licensed and unlicensed recreational marijuana outlets. The marijuana outlet database and the longitudinal dataset from the larger study will provide an unprecedented opportunity to understand whether the proliferation of recreational marijuana outlets and signage/ads in young adults? neighborhoods are related to their increased marijuana use. In analyses, we control for a number of relevant factors that may also account for increased marijuana use over time, such as age and demographic factors, density of and proximity to medical marijuana outlets around one?s home, possession of a provider?s medical marijuana recommendation, prior trajectories of marijuana use over nine annual timepoints, and personal and neighborhood sociodemographic risk factors that are associated with use and location of marijuana outlets, such as number of local alcohol outlets and participant socioeconomic status. Findings can help shape policies as states develop guidelines regarding whether to legalize recreational marijuana for sale, and if so, where to allow marijuana outlets to be located.